The present invention relates generally to tractor hitches which provide convergence of implement loads in agricultural tractors and more particularly to an implement hitch for an articulated four-wheel drive tractor in which convergence occurs substantially at the rear axle.
In the past, as tractors have become larger, the old hitch configurations have remained substantially unchanged with mere structural upgrading to meet load requirements. With front wheel steered two-wheel drive tractors, the draft link convergence, or the intersection in the tractor body of the lines of force applied at the spherical bearings at the rear end of the draft links, was always approximately at the front axle. This was to assure that when the front wheels were turned for steering on a two-wheel drive tractor, the hitch would cause the implement to follow the front wheels into a turn. This principle of convergence at the front axle was retained with the advent of the four-wheel drive articulated tractors.
In early articulated four-wheel drive tractors, it was quickly discovered that abrupt steering changes would often cause implement failures. It appeared that even minor abrupt articulation of the tractor while towing an implement would cause the implement to "wiggle" so as to impose high lateral impactive loads at the tractor-to-implement connection.
While the source of these lateral impactive loads was not determined, it was found that the implement failures could be reduced by modifying the steering system to prevent abrupt steering changes. Thus, the problem was solved at that time by treating symptoms.
As tractors have become larger, wider and wider implements have been developed and this problem has arisen again.
While developing the tractor hitch of the co-pending application Ser. No. 968,359, extensive modeling was used to evaluate the operation of the combination of an articulated tractor with an extra wide implement combination. During this modeling, it was discovered that the basic principle of draft link convergence at the front wheels may be the cause of the large lateral impactive loads. While there is still some controversy in this area, dynamic modeling appears to indicate that draft link convergence affects implement trailing and turning characteristics such that the implement does not respond to the direction of turn of the front wheels.
The rear wheels on an articulated tractor turn in an opposite direction from the front wheels due to the articulation. This means that when the rear wheels go into a turn, the implement which is secured to the rear section turns in the same direction as the rear section; i.e. the rear section of the tractor and the implement go in a direction different from the front wheels and the main body of the tractor. Thus, with changes in the articulation, the implement is always trying to tear itself away from the tractor.
Once this behavior is appreciated, it is apparent that the ideal convergence will be some place between the rear axis of the rear section of the tractor where the implement will exactly follow, on track, the rear section of the tractor.
Further modeling has indicated that the ideal position is probably between the rear axis and the articulation joint. When the new principle was tested in an actual vehicle, it was determined that the ideal convergence would be at the rear axis on a concrete surface but would move forward as a function of wheel slip in a plowed field because the covergence would then be related directly to the actual moving, turning location of the rear section relative to the ground. Since the wheel slip can vary from 5 to 15 percent or more, it is apparent that the actual position of convergence must be determined emperically based on the most desirable wheel slip conditions for the tractors. Unfortunately, the wheel slip conditions will be different in different places in the world.